1. Field of the Invention
The present invention relates to a method and apparatus for accelerating discrete volumes or slugs of liquid, and more particularly to accelerating slugs of liquid through utilization of energy and mass stored in compression of the liquid in a closed container.
2. Prior Art
There is a need for increased productivity in cutting and breaking hard, strong substances such as rock, pavement and frozen earth. One current method of achieving this end is the use of explosives, usually placed in laboriously drilled holes and cavities. The process is noisy, dangerous, and is a batch, as opposed to a continuous, process that is typically slow and expensive. Another method utilizes the mechanical impact breaker, typified by the familiar jackhammer. Such devices are well-developed and in widespread use, but are heavy, punishing to the operator, and break rock too slowly.
Yet another method of breaking and cutting hard, strong substances, but one which is not yet in wide use, utilizes a pulsed liquid jet. A pulsed liquid jet can briefly attain very high jet power for moderate connected power, by storing energy over a time period that is long compared to the jet duration. Such jets are well known to the prior art and typically reach velocities of several thousand feet per second and stagnation pressures to several hundred thousand pounds per square inch. Experimental single-shot laboratory results of several investigators have demonstrated the effectiveness of such pulsed jets for breaking and cutting difficult substances such as pavement and rock.
Pulsed jet devices preferably use a "cumulation" nozzle, such as that disclosed, for instance, in U.S. Pat. No. 3,343,794 to Voitsekhovsky, in which an energetic slug of liquid is supplied at the entrance of a dry nozzle. The foremost portion of the water slug is greatly accelerated as it travels along the contracting passage which concentrates most of the slug energy into the kinetic energy of a small portion of the fluid slug. The resulting transient liquid jet that exits from the nozzle has a peak stagnation pressure many times higher than the static pressure that occurs anywhere within the nozzle, which is of great practical advantage. The internal shape of the nozzle has a profound effect on the wall pressures that occur within the nozzle as is well known in the prior art as demonstrated by U.S. Pat. No. 3,921,915.
The aforementioned experimental results were for the most part obtained using single-shot laboratory apparatus. A successful commercial apparatus must be capable of sustained production of such pulsed liquid jets at a useful repetition rate under field conditions. Most prior inventions utilizing cumulation nozzles have energized the water slug by impact of a moving mass as disclosed for example, in U.S. Pat. Nos. 3,343,794; 3,412,554; 3,905,552; and 3,921,915. In such devices, the pulse energy available to power the liquid jet is the kinetic energy of the impacting mass which must be accelerated by some means such as gravity, a propellant charge or compressed gas. Means must also be provided to empty the nozzle, replenish the liquid slug and maintain the shape and location of the water slug in preparation for each pulse. Previous inventions typically utilize an intermediary piston or diaphragm between the liquid slug and impacting mass and a valve or diaphragm between the liquid slug and the nozzle entrance. Such diaphragms must be replaced before each pulse and the motion of a valve must be closely synchronized with the impact of the moving mass. An intermediary piston must provide for purging of air from the liquid packet chamber. Material considerations, specifically allowable stress, limit the mass impact velocity. Since kinetic energy is proportional to the product of velocity squared and mass, large values of pulse energy require a large moving mass. The result is a heavy apparatus. In addition, the recoil impulse associated with acceleration of a large mass to a high value of kinetic energy results in a tool that is difficult to control. A proposed alternate means of energizing the liquid is spark discharge as disclosed in U.S. Pat. No. 3,647,137. However, this approach requires the supply and rapid switching of large quantities of electrical energy.
U.S. Pat. No. 3,883,075 suggests yet another method of producing a liquid pulsed jet. Under this approach, a multi-channel nozzle block is rotated in front of an ejector supplied with a continuous flow of pressurized liquid. In effect, the rotating nozzle block chops the continuous liquid stream. Such devices are cumbersome and require careful synchronization of the parts.
In general, the prior art liquid pulsed jet devices are handicapped by excessive weight and mechanical complexity, low pulse energy, or very low repetitive firing rate.